Resin blends based on a poly(phenylene ether) and a styrene resin (hereinafter, the blends are referred to as modified PPE's) have any desired degree of heat resistance in the range of from the heat resistance of the styrene resin alone to that of the poly(phenylene ether) alone according to the blending ratio between the poly(phenylene ether) and the styrene resin. They are excellent in electrical properties, dimensional stability, impact resistance, acid resistance, alkali resistance, low water absorption, low specific gravity, etc. The modified PPE resins can be made flame-retardant without using any halogen compound and antimony trioxide, which are regarded as problematic because of their harmfulness. The resins are hence superior from the standpoints of environment and safety. Furthermore, these resins can be designed to be materials having high strength, high rigidity, and high heat resistance by adding an inorganic filler or inorganic reinforcement, and are used in various applications in all the world. Examples of the applications include parts for electrical/electronic appliances, parts for business machines, various exterior materials, and industrial articles.
In view of such recent circumstances in which modified PPE's are used in various applications all over the world, the modified PPE's to be supplied are desired to be produced in many parts of the world where the resins are consumed. Furthermore, with the recent trend toward size increase in domestic electrical appliances, there is a desire for a material which has excellent thermal stability and satisfactory appearance properties so as to be suitable for use as large molded articles such as, e.g., the housings of large television receivers, copiers, printers, and the like.
Incidentally, the properties of a modified PPE are considerably influenced by the properties of the rubber-modified polystyrene used as a main raw material in combination with a poly(phenylene ether) and by the process used for producing the modified PPE.
It has been known that when a polybutadiene containing 50% or higher cis-1,4 bonds and up to 10% 1,2-vinyl bonds is used for a rubber-modified polystyrene in producing a modified PPE, then excellent composition properties are obtained as compared with the case of using a polybutadiene containing a smaller amount of cis-1,4 bonds (e.g., patent document 1).
However, use of the rubber-modified polystyrene employing such a polybutadiene which is relatively easily available all over the world generally poses problems, for example, that the polybutadiene deteriorates during melt kneading and this reduces impact resistance and chemical resistance. It has therefore been necessary to strictly regulate the heat history in producing a modified PPE, and there have been limitations in mass-producing the resin using a high-rotation-speed extruder at a high shear rate.
In recent years, a modified PPE was developed which employs a rubber-modified polystyrene obtained using a partially hydrogenated conjugated diene rubber in which 5-70% by weight of all double bonds have been hydrogenated. This modified PPE is known to be excellent especially in thermal stability (e.g., patent document 2).
However, partially hydrogenated conjugated diene rubbers have a high glass transition temperature and, hence, there is a fear that these rubbers may adversely influence low-temperature impact resistance. In addition, compared to the rubber-modified polystyrenes heretofore in use, the rubber-modified polystyrene employing a partially hydrogenated conjugated diene rubber has had a problem that the production of the partially hydrogenated conjugated diene rubber is costly and, as a result, the cost of production of the modified PPE is high.
A technique for improving low-temperature impact resistance is known in which a rubber-modified polystyrene employing a conjugated diene rubber which has 90% or higher cis-1,4 bonds and a low glass transition temperature is used in combination with a rubber-modified polystyrene employing a partially hydrogenated conjugated diene rubber in which 5-70% by weight of all double bonds have been hydrogenated (e.g., patent document 3).
However, deterioration of the conjugated diene rubber having 90% or higher cis-1,4 bonds still proceeds and, hence, it has been necessary to strictly regulate the heat history in producing a modified PPE. It has been difficult to mass-produce the resin using a high-rotation-speed extruder at a high shear rate.
On the other hand, a process generally employed for producing a modified PPE comprises using an extruder, preferably a twin-screw extruder, to conduct melt kneading with heating. Recently, a high-rotation-speed extruder for mass production is coming to be mainly used.
Hitherto, modified PPE's have generally been produced in such a manner that a poly(phenylene ether), a styrene resin, and other additives are fed en bloc and these ingredients are melt-kneaded with relatively low shearing.
A technique was recently proposed which comprises a first stage in which an intermediate composition comprising a poly(phenylene ether) and a styrene resin and having a relatively high poly(phenylene ether) concentration is produced and a second stage in which the intermediate composition is melt-kneaded together with a styrene resin to produce a target poly(phenylene ether) resin composition (e.g., patent documents 4 to 7).
However, these production processes do not sufficiently reconcile excellent material properties with stable supply, and have been not always satisfactory with respect to various appearance failures such as black foreign particles, unmelted matter, and color unevenness and to practical properties such as stability to high-temperature residence and heat exposure resistance. Furthermore, those processes and compositions both have been insufficient with respect to the supply of a material conforming to global standards which is a recent important demand of customers, i.e., the stable supply of a modified poly(phenylene ether) PPE stably having excellent same properties to customer bases in all parts of the world.
Patent Document 1: JP-A-47-39456
Patent Document 2: JP-A-03-143953
Patent Document 3: JP-A-06-032975
Patent Document 4: JP-A-04-117444
Patent Document 5: JP-A-07-216100
Patent Document 6: JP-A-08-134261
Patent Document 7: JP-A-10-292053